What are the guidelines for referring a patient for a Positron Emission Tomography (PET) scan with 18F-Fluorodeoxyglucose (18F-FDG)?

Writing a Referral for PET Scan with 18F-FDG

Clinical Indications for PET Scan Referral

When writing a referral for an 18F-FDG PET scan, you must specify the clinical indication, as appropriate use varies significantly by disease context. The most evidence-based indications include oncologic staging/restaging, evaluation of plasma cell disorders, assessment of large vessel vasculitis, prosthetic valve endocarditis, and differential diagnosis of dementia when standard workup is inconclusive 1, 2.

Key Information to Include in Your Referral

Patient Preparation Requirements:

• Document that the patient must fast for at least 4-6 hours prior to the scan 1
• Blood glucose levels should be <7 mmol/L (126 mg/dL) preferred, <10 mmol/L (180 mg/dL) acceptable 1
• List all current medications, as certain drugs can interfere with FDG uptake 1
• Note any recent trauma or surgery, as bone fractures can cause false positive uptake for at least one month 1

Essential Clinical Information:

• Primary indication: Specify whether for diagnosis, staging, restaging, or therapy assessment 1
• Relevant history: Include prior imaging results (CT, MRI), biopsy results, and treatment history 1
• Timing considerations: Note if patient has had recent valve surgery (<3 months), as this causes false positive uptake 1
• Antibiotic use: Document if patient is on prolonged antibiotics, which can reduce sensitivity in infectious/inflammatory conditions 1

Disease-Specific Referral Considerations

For Plasma Cell Disorders (Multiple Myeloma):

• FDG PET/CT is indicated for diagnosis, staging, prognostication, therapy assessment, and restaging 1
• Note that false negatives can occur in low glucose metabolism cases 1
• Consider alternative tracers like [11C]MET PET/CT if FDG PET is negative but clinical suspicion remains high 1

For Dementia Evaluation:

• FDG PET should only be ordered after baseline clinical evaluation by a cognitive disorders specialist and structural brain imaging (MRI preferred or CT) have been completed but diagnosis remains unclear 1, 2
• Specify suspected diagnosis (e.g., Lewy Body Dementia shows occipital hypometabolism with "cingulate island sign") 2
• FDG PET is preferred before proceeding to DaTscan due to cost considerations and high diagnostic probability 1, 2

For Large Vessel Vasculitis:

• Imaging should extend from top of head to at least mid-thigh, preferably below knees 1
• Interval between FDG infusion and image acquisition should be at least 60 minutes, preferably 90-120 minutes 1
• Patient positioning: supine with arms down 1

For Prosthetic Valve Endocarditis:

• FDG PET/CT should be performed early, before prolonged antibiotic therapy reduces sensitivity 1
• Note that recent valve implantation (<3 months) causes false positive uptake 1
• Patient must adhere strictly to dietary instructions to suppress myocardial FDG uptake 1
• Results should be discussed in multidisciplinary "Endocarditis Team" including cardiologist, surgeon, infectious disease specialist, and nuclear medicine physician 1

Important Caveats to Document

False Positive Causes:

• Recent bone fractures (require at least one month to normalize) 1
• Osteosynthetic material and bone-metal interface 1
• Inflammatory conditions (arthritis, tendonitis, osteophytes) 1
• Recent surgical adhesives (e.g., BioGlue) 1
• Degenerative skeletal changes in elderly patients 1

Radiation Exposure:

• Total effective dose approximately 10 mSv (6-7 mSv from FDG, 2-4 mSv from low-dose CT) 3
• Avoid in pregnancy unless vital; postpone if possible 3
• Breastfeeding interruption not necessary, but avoid close contact with small children for several hours post-injection 3

Referral Template Structure

Include these sections:

1. Patient demographics and clinical summary
2. Specific indication for PET scan (diagnosis/staging/restaging/therapy assessment)
3. Relevant prior imaging and results
4. Current treatment status (medications, recent surgery, antibiotics)
5. Clinical question to be answered (e.g., "assess for metastatic disease," "differentiate dementia subtype")
6. Contraindications or special considerations (pregnancy, recent trauma, glucose control issues)
7. Urgency level and preferred scheduling timeframe

The referral should enable the nuclear medicine physician to determine appropriate imaging protocol, interpret findings in clinical context, and provide actionable recommendations for management 1.